One of the greatest warnings George Orwell intended to give in 1984 concerns the dangers of losing the ability to think for one's self. Throughout the book, Orwell equates true liberty with the ability to think. Slavery ensues when a person's abilities to think and to emote are suppressed.
The warning against the suppression of thought is especially seen in Orwell's protagonist Winston Smith. Winston is an intellectual who works in the Ministry of Truth; his job is to change the history records to match the Party's objectives. However, due to his intelligence, Winston cannot help being reflective and analytical. As a deep thinker, he cannot help but begin to question the correctness of the Party and even starts relaying his criticisms in a secret diary. Orwell is using Winston's ability to think, analyze, and criticize to represent Winston's liberation even though that liberation is, sadly, only temporary.
The Party suppresses thoughts and the ability to emote, thereby enslaving Winston and all citizens, through various laws. The Party even goes so far as to make it illegal to think thoughts contrary to the Party in one's sleep, and citizens can be arrested for talking in one's sleep. Even facial expressions and gestures that express emotions that do not align with the Party are illegal, as seen in the following passage:
The smallest thing could give you away. A nervous tic, an unconscious look of anxiety, a habit of muttering to yourself--anything that carried with it the suggestion of abnormality, of having something to hide. In any case, to wear an improper expression on your face (to look incredulous when a victory was announced, for example) was itself a punishable offense. There was even a word for it in Newspeak: facecrime, it was called. (Chapter V)
Hence, as we can see, Orwell is using his concepts of "thoughtcrime" and "facecrime" to warn that true freedom can only be obtained so long as people maintain their ability to think for themselves.